选择查询 - 将同一个表多次连接到不同的列

Question

我是一个SQL新手，试图编写一个涉及多个表联接的select查询，多次使用一个表。

关系如下所示：

我想要实现的查询的输出是：

Truck_ID|Pushbutton_ID|Trailer01_ID|Trailer02_ID|Trailer03_ID|Trailer04_ID|Trailer05_ID

表格Trucks和Pushbutton_Assignments的加入非常简单。难点是多次加入Trailer_Assignment表。

Trailer_Assignments的结构是：

Trailer_ID|Truck_ID|Order

Trailer_ID和Truck_ID字段与其他表具有外键关系。查询的TrailerXX_ID字段与Order表的Trailer_Assignment字段相关;所有Order == 1的记录都有Trailer_ID Trailer01_ID，Order == 2到Trailer02_ID的记录等等。

我基于here的查询。它适用于单个列，但Access（我认为）不像后续列。

工作查询代码（单列）：

SELECT Trucks.ID, Pushbutton_Assignment.Pushbutton_ID, Trailer_Assignment.Trailer_ID AS Trailer01_ID, Trailer_Assignment.Trailer_ID AS Trailer02_ID, Trailer_Assignment.Trailer_ID AS Trailer03_ID, Trailer_Assignment.Trailer_ID AS Trailer04_ID, Trailer_Assignment.Trailer_ID AS Trailer05_ID FROM (Trucks LEFT JOIN Pushbutton_Assignment ON Trucks.ID = Pushbutton_Assignment.Truck_ID) LEFT JOIN Trailer_Assignment ON Trucks.ID = Trailer_Assignment.Truck_ID AND Trailer_Assignment.Order = 1;

编辑：

非工作查询代码（两列）：

SELECT Trucks.ID, Pushbutton_Assignment.Pushbutton_ID, Trailer_Assignment.Trailer_ID AS Trailer01_ID, Trailer_Assignment.Trailer_ID AS Trailer02_ID, Trailer_Assignment.Trailer_ID AS Trailer03_ID, Trailer_Assignment.Trailer_ID AS Trailer04_ID, Trailer_Assignment.Trailer_ID AS Trailer05_ID (((FROM (Trucks LEFT JOIN Pushbutton_Assignment ON Trucks.ID = Pushbutton_Assignment.Truck_ID)) LEFT JOIN Trailer_Assignment ON (Trucks.ID = Trailer_Assignment.Truck_ID AND Trailer_Assignment.Order = 1)) LEFT JOIN Trailer_Assignment ON (Trucks.ID = Trailer_Assignment.Truck_ID AND Trailer_Assignment.Order = 2));

这是使用Access 2003。

Answer 1

MS Access具有各种奇怪的限制，例如在LEFT JOIN条件下难以处理常量。所以，只需投降并使用相关的子查询。 。 。你只需要一个值：

select t.id, pba.pushbutton_id,
       (select top 1 ta.trailer_id from Trailer_Assignment as ta where ta.Truck_id = t.id and ta.order = 1
       ) as Trailer01_id,
       (select top 1 ta.trailer_id from Trailer_Assignment as ta where ta.Truck_id = t.id and ta.order = 2
       ) as Trailer02_id,
       (select top 1 ta.trailer_id from Trailer_Assignment as ta where ta.Truck_id = t.id and ta.order = 3
       ) as Trailer03_id,
       (select top 1 ta.trailer_id from Trailer_Assignment as ta where ta.Truck_id = t.id and ta.order = 4
       ) as Trailer04_id,
       (select top 1 ta.trailer_id from Trailer_Assignment as ta where ta.Truck_id = t.id and ta.order = 5
       ) as Trailer05_id
from Trucks as t LEFT JOIN
     Pushbutton_Assignment as pba
     on t.id = pba.truck_id;

Answer 2

由于您在查询中多次使用同一个表，因此需要为每个表提供一个别名：

SELECT Trucks.ID, 
   Pushbutton_Assignment.Pushbutton_ID,
   ta1.Trailer_ID AS Trailer01_ID,
   ta2.Trailer_ID AS Trailer02_ID,
   ta3.Trailer_ID AS Trailer03_ID,
   ta4.Trailer_ID AS Trailer04_ID,
   ta5.Trailer_ID AS Trailer05_ID
FROM (Trucks LEFT JOIN Pushbutton_Assignment ON Trucks.ID = Pushbutton_Assignment.Truck_ID) 
    LEFT JOIN Trailer_Assignment ta1 ON Trucks.ID = ta1.Truck_ID AND ta1.Order = 1
    LEFT JOIN Trailer_Assignment ta2 ON Trucks.ID = ta2.Truck_ID AND ta2.Order = 2
    LEFT JOIN Trailer_Assignment ta3 ON Trucks.ID = ta3.Truck_ID AND ta3.Order = 3
    LEFT JOIN Trailer_Assignment ta4 ON Trucks.ID = ta4.Truck_ID AND ta4.Order = 4
    LEFT JOIN Trailer_Assignment ta5 ON Trucks.ID = ta5.Truck_ID AND ta5.Order = 5;

Answer 3

SELECT Trucks.ID, 
   Pushbutton_Assignment.Pushbutton_ID,
   iif(ta.order = 1, ta.Trailer_ID, NULL) AS Trailer01_ID,
   iif(ta.order = 2, ta.Trailer_ID, NULL) AS Trailer02_ID,
   iif(ta.order = 3, ta.Trailer_ID, NULL) AS Trailer03_ID,
   iif(ta.order = 4, ta.Trailer_ID, NULL) AS Trailer04_ID,
   iif(ta.order = 5, ta.Trailer_ID, NULL) AS Trailer05_ID
FROM Trucks LEFT JOIN Pushbutton_Assignment 
ON Trucks.ID = Pushbutton_Assignment.Truck_ID 
LEFT JOIN Trailer_Assignment ta ON Trucks.ID = ta.Truck_ID 

这是使用if语句执行此操作的另一种方法。您只需要参考一次表格。如果需要，添加where子句，仅查找订单1,2,3,4和5。